Conventional technologies for amplifying signal light include erbium doped fiber amplifiers (EDFAs) and optical parametric amplifiers (OPAs) employing a non-linear optical medium. The EDFAs have a gain band that is dependent on a doped rare-earth element (Er). By contrast, the optical parametric amplifier, whose gain band is secured by adjusting a zero-dispersion wavelength of an optical fiber, is capable of obtaining a wide gain bandwidth (see, e.g., Ho, M-C., et al, “200-nm-Bandwidth Fiber Optical Amplifier Combining Parametric and Raman Gain,” J. Lightw. Technol, 19, pp. 977-981, 2001) and a high gain volume (see, e.g., Torounidis, Thomas, et al, “Fiber-Optical Parametric Amplifier With 70-dB Gain,” IEEE Photon. Technol. Lett, 18, pp 1194-1196, 2006). The optical parametric amplifier is capable of realizing a low noise figure (see, e.g., Tong, C., et al., “Measurement of Sub-1 dB Noise Figure in a Non-Degenerate Cascaded Phase-Sensitive Fibre Parametric Amplifier,” 35th European Conference on Optical Communication, Paper 1.1.2, 2009).
The EDFA, which amplifies the signal light by a stimulated emission process having a relatively slow relaxation time, does not change the waveform of the signal light even in the range of input power with gain saturation. By contrast, the optical parametric amplifier, which amplifies the signal light by a high-speed parametric process, produces a non-linear output relative to the intensity of the signal light waveform in the range of input power with gain saturation and therefore, can be used as a waveform shaper as well.
The optical parametric amplification is realized by combining the signal light and pump light of a wavelength near the zero-dispersion wavelength of the non-linear optical medium and inside the non-linear optical medium, amplifies the signal light by the optical parametric process. The optical parametric amplifier is provided with, for example, the pump light of a wavelength different from that of the signal light; an optical coupler that combines the signal light and the pump light; the non-linear optical medium; and an optical filter that takes out the signal light.
The conventional technologies described above, however, optical signals which are allocated at an anomalous dispersion wavelength of the non-linear optical medium is deteriorated by increase of its noise due to the phenomenon of modulation instability (non-linear optical effect). This causes a problem in that noise increases and the signal quality deteriorates at the time the signal light is amplified. The increase in noise caused by the optical parametric amplification will be specifically described.
FIGS. 8A to 8D depict an increase in noise due to optical parametric amplification. In FIGS. 8A to 8D, the horizontal axis represents the wavelength (λ) and the vertical axis represents the optical power. As depicted in FIG. 8A, in the optical parametric amplifier, signal light 801 and pump light 802, respectively of wavelengths differing from each other, are combined and input to the non-linear optical medium. A wavelength λs represents the wavelength of the signal light 801. A wavelength λp represents the wavelength of the pump light 802.
As depicted in FIG. 8B, when the signal light 801 and the pump light 802 are input to the non-linear optical medium, idler light 803 is generated by four-wave mixing as a high-order effect. Wavelength λi represents the wavelength of the idler light 803. Since the wavelength λi of the idler light 803 is the anomalous dispersion wavelength, the noise of the idler light 803 increases, as depicted in FIG. 8C.
For this reason, the idler light 803 having increased noise modulates the intensity of the pump light 802 and the pump light 802 comes to have great noise, as depicted in FIG. 8D. The pump light 802 having great noise modulates the intensity of the signal light 801 and therefore, the noise of the signal light 801 increases. Thus, since the optical parametric amplification generates the idler light 803 in a non-linear optical medium, the noise of the signal light 801 increases due to modulation instability (non-linear optical effect).